An infinite group of spread one
An explicit subgroup of Thompson’s group F is infinite and has ordinary spread exactly one.
Scope
The subgroup consists of maps with equal endpoint slopes; the spread condition holds exactly for k ≤ 1. The key generation argument specializes Gili Golan-Polak’s published theorem.
Formal record
- Lean statement
- Kourovka/Problems/P21_38/Statement.lean
- Lean proof
- Kourovka/Problems/P21_38/Solution.lean
- Endpoint
- Kourovka.P21_38.notebookStatement
- Source commit
- 5a6b2c18e326b7b0281f00b64cade629acbfe1f5
- Verification
- Integrity passed (verification record)
- Statement review
- Independent review
- Novelty
- No earlier complete solution located in a bounded search (report); no priority claim is made
- Human acceptance
- 16 September 2026, human statement verifier (acceptance record)
- Agent version
- Nilradical v0
Strict verification passed on 16 September 2026 for 7 endpoints: fresh compilation, frozen-statement comparison, a three-axiom policy, and checks by both Lean and Nanoda. Independent statement review passed. Final human statement acceptance was recorded on 2026-09-16 for the linked proof snapshot.
All 551 inventory entries matched the published mathematical source snapshot. Each result has a separate frozen specification and strict verification receipt.
Credit
Construction, proof and formalization by Nilradical v0. The generation argument builds on Gili Golan-Polak’s theorem.
- Nilradical v0 — discovery, proof and formalization
- C. Donoven and S. Harper — original question
- Gili Golan-Polak — prior generation theorem
- Group-approximation contributors — adapted formal foundations
Note
Read the mathematical note, an agent-written account of the result and its proof. It is informal and not refereed.
Cite
Nilradical. Nilradical v0: solution and Lean proof of Kourovka Problem 21.38. Repository snapshot 5a6b2c18e326.
@misc{nilradical_v0_kourovka_21_38,
author = {{Nilradical}},
title = {Nilradical v0: solution and Lean proof of Kourovka Problem 21.38},
year = {2026},
howpublished = {Mathematical result with Lean formalization},
url = {https://github.com/alunik/kourovka-lean/blob/5a6b2c18e326b7b0281f00b64cade629acbfe1f5/Kourovka/Problems/P21_38/README.md},
note = {Source commit 5a6b2c18e326b7b0281f00b64cade629acbfe1f5; Nilradical v0}
}